Method for continuous casting of rimming steel



United States Patent 3,411,897 METHOD FOR CONTINUOUS CASTING OF RIMMINGSTEEL Paul E. Lindberg, Jr., Grosse Ile, Mich., assignor, by mesneassignments, to Concast Incorporated, New York N.Y., a corporation ofDelaware N0 Drawing. Filed Sept. 8, 1965, Ser. No. 485,915 7 Claims.(Cl. 75-129) ABSTRACT OF THE DISCLOSURE This is a method of partiallydeoxidizing molten steel to prepare rimming steel to be continuouslycast. Vanadium is added to molten steel in an amount not more than about82.50 pounds per 47 tons of molten steel. Thereafter not more than atotal of about 50 pounds of aluminum are added per 47 tons of the moltensteel, the aluminum being added a portion at a time to a point at whichthe customary agitation of the molten steel is without boiling andfrothing when the steel is poured. The molten steel is then continuouslycast by pouring it in one end of an open-ended mold and withdrawing apartially solidified casting from the other end.

This invention relates to the continuous casting of steel and, moreparticularly, to the continuous casting of rimming steel.

In the continuous casting of steel, molten steel is poured into theupper end of a mold open at its bottom. The mold is cooled by a liquid,such as water, circulated in the mold walls. When pouring is commenced,the bottom of the mold is closed with a metal plug. The plug cools themolten metal and the metal fuses to the plug. The cooled mold cools themetal in contact with the mold causing the molten metal to solidify andform a skin or sheath around the molten metal in the mold. The plug iswithdrawn from the bottom of the mold and the steel, fused to the plug,follows. A continuous slab or casting of steel is thus formed. The plugis later removed from the end of the slab.

The skin or sheath formed around the molten metal in the mold isrelatively thin at the time the cast slab emerges from the bottom of themold. The metal, still molten in the center of the slab, is contained inthe slab by this skin. The slab is supported by rollers which contactthe slab below the mold. Some of the rollers are driven to regulate thespeed at which the slab leaves the mold.

After leaving the mold, the cast slab is cooled. As the slab is cooled,the molten metal in the slab progressively solidifies from the outerwall of the slab inwardly. The skin or wall of solid metal thickensuntil, finally, all the metal in the slab is cooled and solidified. Thepoint where the entire slab is solidified is below the mold and may bethirty-five feet, or more, from the point where the slab leaves themold. Thus, as the slab leaves the mold, and for a considerable distancebelow the mold, the skin of solid metal around the slab supports themolten metal in the slab. Obviously, if this skin should be ruptured,the molten metal will escape and considerable damage to operatingequipment can result, to say nothing of failure to form the requiredslab.

Unlike the casting of ingots where the metal is confined in a mold untilsolidified and undesired oxides and impurities migrate to the top andbottom of the ingot and can be cut from the ingot during rolling, incontinuous casting these impurities remain in the cast. Among these isuncombined oxygen which, as the molten metal cools, reacts and escapesas a gas from the molten metal. This escaping gas agitates the moltenmetal. Where the gas fails to escape before the metal solidifies, abubble or void of substantial size may be formed in the cast.

A substantial portion of the molten metal in continuous casting iscooled and solidified after the slab leaves the mold. Where thecollecting gas can escape upward through the molten metal, the gasvigorously agitates, boils and froths the molten metal. Such vigorousagitation and boiling interferes with the desired cooling of the moltenmetal by the cooled mold wall and hinders proper formation of the skinof solid metal around the slab, vital to continuous casting. Failure ofthe skin to properly form results in a weak wall which cannot supportthe molten metal.

The uncombined oxygen can be removed from the metal before thecontinuous cast is poured. This can be accomplished by the addition ofmaterials to the molten metal which will react with, and absorb, all orsubstantially all of the uncombined oxygen.

Where rimming steel is to be formed, however, a controlled amount ofuncombined oxygen is required. To obtain the necessary rimming action,through controlled agitation of the molten metal in the continuous cast,a certain amount of uncombined oxygen is desirable in the molten metal.This uncombined oxygen reacts to form a gas which moves through themolten metal and agitates the metal. By regulating the uncombinedoxygen, the agitation or movement of the molten metal can be controlledso that the metal is agitated in a pattern in the mold. Thus, a cast ofrelatively dense, pure metal solidifies from the surface of the castinwardly and impurities are moved away from the surface of the cast.

Attempts to adapt ingot casting deoxidation techniques for rimming steelto continuous casting have met with limited success and have resulted inthe formation of relatively thin rimmed areas, high proportions of oxideimpurities and casts which, when rolled, produced rolled sheets withstreaks and flaws on, or close to, the surfaces of the sheets.

One of the objects of the instant invention is to provide an improvedmethod for continuous casting of rimming steel.

A further object is to provide such a method that will form a relativelythick rimmed area.

A further object is to provide such a method wherein impurities in thecast are moved away from the surface of the cast.

Still a further object is to provide a continuous cast which can berolled into relatively thin sheets without impurities or flaws on thesurfaces of the sheets.

These and other objects will be more apparent from the followingdescription:

In the instant method the metal for casting is prepared in the customarymanner, is poured into a ladle and, from the ladle, is poured into thetundish of the continuous casting mold. At the time the metal is readyfor pouring into the ladle, the iron oxide content of the slag ismeasured in accordance with customary practices.

The molten metal is poured into the ladle at a temperature of about 2950to 3060 F. Difficulties in sampling, and the time available, do notallow precise measurement of the iron oxide content of the slag.However, and as will be more apparent from the following description,such measurement has been found sufficient for purposes of the instantinvention. In addition to measuring the iron oxide content of the slag,the carbon, manganese, phosphorus and sulfur content of the metal arealso measured. These later measurements are primarily for the purpose ofdetermining the additions to the metal which may be required to producethe desired steel but may have an indirect relationship to the oxygencontent.

The following description is based on an average charge of 47 tons ofmolten metal. Any substantial change in weight would, of course, lead tocorresponding changes in the ladle additions to be described.

For rimming purposes, it is desirable to control the carbon content ofthe molten metal to be cast. The initial carbon content of the moltenmetal is regulated by the oxygen blowing of the metal. This blowing isadjusted so that the carbon content of the molten metal, when ready forpouring for low carbon rimming steel, does not substantially exceed0.07%. Where higher carbon rimming steel is to be continuously cast, theblowing is adjusted for a higher carbon content. Where excess carbon ispresent, oxygen blowing is continued until the carbon content isreduced.

Where the carbon content of the metal is 0.07% or less, ten pounds ofcarbon, in the form of breeze, are added to the ladle as the metal ispoured. If the metal has been over oxidized and the carbon content issubstantially less than 0.05%, carbon addition may be increased totwenty pounds.

Carbon added to the molten metal, whether as breeze or in alloyadditions, does, to a certain extent, react with and remove as a gassome of the uncombined oxygen in the molten metal. The amount of carbonthat can be added, however, is limited. Thus, in the instant invention,the balance of the uncombined oxygen removed to attain a substantialuniform pattern of agitation in the molten metal during continuouscasting and, at the same time, avoid excess agitation, boiling and frothin the mold and cast is removed by reaction with vanadium and aluminumadditions to the molten metal.

Aluminum is well-known as a deoxidant in steelmaking. The aluminumcombines with the uncombined oxygen forming aluminum oxide. In ingotcasting of rimming steel, the aluminum oxide, which when formed is animpurity, migrates to the top of the ingot and is cut from the ingotduring rolling. Attempts, heretofore, to utilize aluminum as a deoxidantin continuous casting of rimming steel have resulted in erratic rimming,relatively thin rims, and the formation of relatively large agglomeratesof aluminum oxide at or near the surface of the cast slab which, whenthe slab is rolled, resulted in streaks and flaws on the surfaces of therolled sheet.

In the instant invention, it has been discovered that the formation ofthese agglomerates of aluminum oxide at or near the surface of thecontinuously cast slab can be avoided by utilizing a combination ofvanadium and aluminum as the deoxidant. It has been further discoveredthat, when so used for this purpose, the amount of each materialemployed is critical to the formation of a well rimmed cast free ofoxide flaws at or nearthe surface.

The amount of deoxidant addition depends, of course, on the oxygencontent of the metal for which the iron oxide content of the slag in thesteelmaking furnace provides a satisfactory estimate for presentpurposes.

Vanadium, in the form of ferro vanadium, has been found to beacceptable. In the following description and examples ferro vanadiumhaving an average analysis of 55.00% vanadium, 4.00% silicon, 2.50%carbon, and the balance i on s emp y In carrying out the instantinvention, it has been discovered that the amount of ferro vanadium andaluminum which can be added to the molten metal can be varied over awide latitude. However, there is a limit on the maximum amount of eachmaterial that can be added and still obtain the desired results. Forexample, it has been found that the addition of more than pounds offerro vanadium (82.50 pounds vanadium) to a 47 ton melt does not producethe desired results. Likewise, it has been found that the addition ofmore than 50 pounds of aluminum does, not produce the desired results.Within these maximum limits, however, the amounts added can be variedprovided, of course, that a sufiicient amount of the combined materialsis added to attain the required deoxidation.

In the practice of the invention, the ferro vanadium is added tostabilize the melt and the aluminum to control the rimming action. Theamount of ferro vanadium to be added to a particular melt is determinedfrom the iron oxide content of the slag and the chemistry of the metal.It is preferred to add the ferro vanadium in a fixed amount and to varythe aluminum addition to suit the needs of the particular melt. Forexample, seventy-five pounds of ferro vanadium are added to the 47 tonmelt and suflicient aluminum is added to reduce the uncombined oxygencontent of the melt to the desired level. Where the measured iron oxidecontent of the slag is measured as 16% or less, seventy-five pounds offerro vanadium is sutficient to maintain the required addition ofaluminum well under the 50 pound limit. Where the iron oxide measurementexceeds 16% the ferro vanadium addition may be increased to one hundredpounds if it is desired to maintain the aluminum addition at a lowlevel.

In the process of the instant invention, the vanadium apparently acts asa buffer and lowers the uncombined oxygen content of the molten metal toa point where it can be effectively controlled by the aluminum addition.Final control of the uncombined oxygen content is regulated "by thealuminum addition. Because the measurement of the iron oxide content ofthe slag and the residual elements in the metal are not precisemeasurements, and the amount of uncombined oxygen in the metal may nothave been precisely determined, the ferro vanadium, is first added tothe ladle. The aluminum is then added and the amount of aluminum isdetermined, to some extent, by the activity of the tapping stream andthe metal in the ladle.

One method which has been found effective for regulating the aluminumaddition and at the same time attaining the desired results of theinstant invention, is to estimate the amount of aluminum required fromthe iron oxide measurement and the ferro vanadium addition and then adda portion of the estimated aluminum. If the metal in the ladle isrelatively calm, is not excessively boiling and there is no frothing,the remaining portion is not added. If, on the other hand, the metal inthe ladle remains wild, boils excessively and froths, the additionalaluminum is then added. In some instances, even after the addition ofthe remaining portion further addition of aluminum may be required toreduce the activity of the metal in the ladle.

The melt for continuous casting is prepared at a temperature of about2950 to 3060 F. The temperature of the melt in the ladle is about 2850to 3000 F. and the temperature in the tundish, as the molten metal isbeing delivered to the continuous casting mold is about 2800 to 2875 F.As the temperature drops the molten metal may become more active and theactivity may exceed that necessary for good and effective rimming. Wheresuch activity occurs, additional aluminum may be added in the tundish.Usually, however, tundish addition is not necessary and it is preferredto add all of the required alumium to the ladle.

The following are examples of casts, continuously cast following themethod and process of the instant invention. In each example an averagecharge of 47 tons of molten metal was employed.

Preliminary Analysis ladle Additions FeO FeMn FeV Al NoTE.-C=carbon;Mn=manganese; Fe0=ferrous oxide (calculated from total iron); FcMn=ierromanganese; FeV=ferro vanadium; Al= aluminum.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention, in the useof such terms and expressions, of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is:

1. A method of partially deoxidizing and continuously casting rimmingsteel comprising adding to molten steel not more than about 82.50 poundsof vanadium per 47 tons of molten steel, then adding not more than about50 pounds of aluminum per 47 tons of molten steel, said aluminum beingadded a portion at a time to a point at which agitation of the moltensteel is without vigorous boiling and frothing when poured, thereaftercontinuously casting the steel by pouring it in one end of an open-endedmold wherein the periphery of the steel is solidified to form a castinghaving a molten core, withdrawing the casting from the mold as acontinuous strand, and subsequently cooling the casting to solidify itcompletely with a rimmed structure.

2. The method of claim 1 in which the vanadium is ferro vanadiumcontaining the specified amount of vanadium.

3. The method of claim 1 in which, when the slag on the molten steel,immediately before tapping, contains less than about 16% iron oxide, theamount of vanadium added is from about 27.50 pounds to about 41.25pounds.

4. The method of claim 1 in which, when the slag on the molten steel,immediately before tapping, contains more than about 16% iron oxide, theamount of vanadium added is from about 41.25 pounds to about 82.50pounds.

5. The method of claim 1 in which the amount of aluminum added is fromabout 10 pounds to about 50 pounds.

6. The method of claim 1 in which the amount of vanadium added is fromabout 27.50 pounds to about 82.50 pounds.

7. The method of claim 1 in which the amount of vanadium added is fromabout 27.50 pounds to about pounds, and the amount of aluminum added isfrom about 10 pounds to about 40 pounds.

References Cited UNITED STATES PATENTS 1,420,328 6/1922 McConnell 1291,727,180 9/1929 Saklatwalla 75129 X 2,221,781 11/1940 Critchett et al.7558 2,253,574 8/ 1941 Norbeck 755 8 2,291,842 8/ 1942 Strauss 75582,360,271 10/ 1944 Peterson 7558 X 2,999,749 9/ 1961 Saunders et al 75583,030,204 4/1962 Staggers et al 75129 L. DEWAYNE RUTLEDGE, PrimaryExaminer.

H. W. TARRING, Assistant Examiner.

